This study will compare the clinical periodontal status and microbial assessments of subgingival flora before, and at 1, 3, and 6 month intervals following systemic treatment with metronidazole in subjects with periodontitis. Thirty volunteers who require 1-week treatment with metronidazole for non-dental infections, and 10 controls matched according to age and periodontal status, who meet the following criteria, will be selected for this study: at least 30 yrs. of age; no current pregnancy; no recent antibiotic therapy for the control group, and no recent antibiotic therapy other than the 1-week metronidazole treatment for the experimental group; no history of diabetes or blood dyscrasias; no oral prophylaxis within the last 6 months; at least one periodontal pocket equals 5 mm. Two sites will be studied per subject. The sites will be selected so that 10 will be assigned to one of each of 3 disease state categories: slight, moderate, or advanced periodontitis based on connective tissue attachment loss and pocket depth. At each site prior to collecting the subgingival plaque sample, the Gingival Index, pocket depth, attachment loss, bleeding tendency, and Periodontal Disease Index will be recorded. The percentage of spirochetes of spirochetes and other motile organisms will be determined by phase microscopy and the total colony forming units will be determined by cultures on selective media for anaerobes. Two investigators will perform all clinical examinations, one of whom also will collect and code plaque samples. Two other investigators will perform all phase microscopic assessments. One staff research assistant will make bacteriologic cultures and assessments.